Components of Fiber optic transmission
Fiber optics uses pulses of light to transmit data. It is convention to have a pulse of light indicate a 1 and absence of light indicate a 0. Set up for fiber optic communication includes three main components: a light source, a transmission medium, and a detector. Together, these components create a complete unidirectional transmission system.
The light source creates a pulse of light. Commonly used light sources include LEDs, semiconductor lasers, and photo diodes. The detector recognizes light pulses by generating an electrical pulse when light hits it.
The transmission medium is a thin fiber of glass encased in another medium that guides the light pulse to its destination. (see figure 1) Light is kept inside the transmission medium due to the physical principle of refraction. Refraction deals with the change in direction of a light ray as it passes from one medium to another. Under the right conditions, all of the light will be reflected back into the original medium, the glass fiber.
Figure 1 composition of a fiber optic cable
The physics of light refraction
When a beam of light passes from one medium to another, part of the light is reflected at the incident angle, and part of the light enters the new medium at a different angle. Refraction is the change in direction of the beam. For example, when light enters glass from air, it is bent towards the normal line like so:
The magnitude of this change in direction is related to the incident angle and the indexes of refraction of the two mediums. An index of refraction is a physical property of a material that can be experimentally determined. The index of refraction is defined as the ratio of the speed of light in a vacuum to the speed of light in a material. Because light travels the fastest in a vacuum, the index of refraction is always greater that 1.
The angle of refraction can be determined by Snell's law:
where n1 and n2 are the indexes of refraction of the two mediums, q1 is the incident angle, q2 and is the angle of refraction (see figure 3). This relation was first discovered experimentally, but it can be shown mathematically as well.
When q2 is greater than 90°, all of the light is reflected back into the initial medium, and none is refracted. This situation is called total internal reflection. Using the geometry and Snell's law, we note that total internal reflection occurs when the incident angle is greater than qc, where
Here, qc is the incident
angle that yields a 90° refraction angle.
(see figure 4)
In a fiber optic cable, when there is not total internal reflection of a ray, light is lost each time the ray hits the core-cladding boundary. The further it travels, the weaker the signal will get, and eventually it will not be detectable. When total internal reflection does occur, there is no loss of light due to refraction and the signal can travel significantly longer distances.
Therefore, having a large angle of incidence and the right proportion of the indexes of refraction can lead to all the light being internally reflected, thus staying in the glass fiber transmission medium. The core of a fiber optic cable is surrounded by cladding which is also made of glass (see figure 1). The inner portion of glass has a higher index of refraction than the cladding so that the light stays in the core. In order to create a large angle of incidence, the light pulse is directed almost parallel to the axis of the fiber, and the fiber is not given sharp turns.
Multi-mode vs. Single Mode
Each pulse of light is composed of different rays with varying angles of incidence on the core-cladding boundary. Thus, one pulse of light will have several rays bouncing around in the core at different incident angles, or different "modes". A fiber that allows this is called a multimode fiber. On the other hand, when the fiber's diameter is really small, on the order of a few wavelengths of light (7 to 9 microns as opposed to 50 to 100 microns for multimode), there is not much room for the rays to bounce around and the light pulse travels straight in the fiber. As the rays in this type of fiber are all traveling in the same direction, it is called single-mode fiber. Single-mode fiber is much higher bandwidth than multimode but it is also more expensive and difficult to maintain.
Attenuation and Dispersion
As light travels through the glass fiber, it looses some energy. Light travels through a material more or less easily depending on it's wavelength. This means that the index of refraction is slightly different for various wavelengths of light traveling in the same medium. This effect is called dispersion. It also means that certain wavelengths of light loose energy at a faster rate than others in a given medium.
The loss of too much energy causes the signal to become too weak to detect. The term for this is attenuation. Attenuation can be minimized by thoughtful choice of the light pulse's wavelength. As a result of this, light with a 0.85, 1.30, or 1.55 micron wavelength is used.
Dispersion causes light pulses to spread out due to the small variations of the wavelength of light in a pulse. Dispersion of a pulse increases the farther it travels. Research os being done to minimize dispersion. One promising solution is using soliton pulses. Soliton pulses have a a special shape related to the reciprocal of the hyperbolic cosine. This shape cancels out the effects of dispersion on the signal. Reaserchers are working to bring soliton pulses out of the laboratory and into practical use.
Step vs. Graded Refractive-Index-Profile Cores
Thus far, we have only considered the index of refraction to be the same throughout the core of the fiber optic cable. This type of core has a step refractive-index-profile. Another option is to have a core with a varying index of refraction. This would utilize a graded refractive-index-profile core.
Figure 5 core refractive index profiles
As shown in figure 5, the graded core has its highest index of refraction at the center, and it diminishes smoothly the further away from the center you go. This type of core can be analyzed as if it were made of of thin layers of glass with different indexes of refraction, decreasing in finite steps from the center. This causes the rays to bend, due to refraction, within the core. Paths of light rays in a graded core can be seen below in figure 6.
Figure 6 ray paths in a graded core
upper right: an unbound ray
upper left and lower: bound rays
This type of core increases the amount of bound rays, as it catches rays with greater initial incidence angles than a core of uniform index of refraction.
Last Modified: 3/15/00
By Brian Patterson and
Erin Quealy